(1) Field of the Invention
The present invention relates to a development process in electrophotography. More particularly, the present invention relates to an improvement in the process for controlling the toner concentration in a developer appropriately based on the optical density of a toner image formed on a photosensitive plate in correspondence to an imitation original.
(2) Description of the Prior Art
In the development process using a two-component type developer comprising a mixture of an electroscopic fixable toner and a magnetic carrier, it is necessary to control the toner concentration (the proportion of the toner) in the developer by some means or other.
This toner concentration should be detected for supplying a toner to the developer, and as means for detecting the toner concentration, there have been practically adopted the process in which the toner level in a development tank is detected, and the process in which the permeability of the developer is detected. However, these processes are defective in that the image density (optical density) of the toner image formed by the development is not well in agreement with the toner concentration in the developer.
Therefore, at the present, there is adopted a process in which the image density of a toner image formed on a photosensitive plate in correspondence to an imitation original is detected and the toner concentration in the developer is controlled based on the detected image density, that is, if the image density is lowered, the toner is supplied into the developer and if the image density is increased, supply of the toner is stopped.
According to this conventional process, the object of maintaining the image density of the toner image on a photosensitive plate at a certain level can be attained, but it was found that the object of maintaining the toner concentration in the developer within an optimum range is not satisfactorily attained.
In fact, the image density of the toner image on the photosensitive plate is seriously influenced by the toner concentration in the developer. However, it was found that in the case where the image density of the toner image is detected and the toner concentration in the developer is controlled based on the detected image density, scattering of toner or tailing in the formed image often takes place.